This invention relates in general to the field of voltage generation, and, more particularly, to bandgap reference voltage generation.
Bandgap reference voltage generation is an important and integral part of integrated circuit (IC) design. Linear, custom, and memory circuits can require temperature and voltage compensation. In addition, logic families of both emitter-coupled logic (ECL) and common-mode logic (CML) require regulators. The function of a regulator in a logic circuit is to provide a reference voltage to supply a regulated potential across a resistor to create a current source for a particular gate. Designs using discrete components were the first to be used as IC regulators.
Particular difficulties occur in fully compensating voltage regulators for temperature and voltage variations. Voltage compensation refers to providing stable output for power supply changes. Temperature compensation refers to maintaining stable output while the voltage regulator experiences temperature fluctuations. Full compensation refers to maintaining stable voltage output under both temperature and power supply changes.
One traditional approach to voltage-compensation of voltage regulators has been to use P-N-P transistors as shunt devices for excess current generated by changes in the power supply. P-N-P transistors, however, are typically difficult to make, unreliable, and have poor current density capability. Additionally, P-N-P transistors have a temperature tracking problem which causes the temperature coefficient associated with the regulated output voltage, V.sub.CS, to be less positive than desired.
Voltage compensation in P-N-P transistor designs was improved by replacing the P-N-P shunt device in the bandgap reference voltage generator with a differential amplifier. Using the reference voltages generated, the differential amplifier operates very similarly to the P-N-P device. While the problem associated with the production of P-N-P transistors is eliminated by using the differential amplifier, there remains a problem with current density tracking over temperature. It is desirable improve the tracking characteristics of the fully compensated differential amplifier voltage regulator throughout a wider range.
Temperature compensation to produce an output voltage, V.sub.CS or V.sub.BB, with essentially zero temperature coefficient, is typically accomplished by summing two voltages having opposite temperature coefficients. The positive temperature coefficient can be produced by two transistors operated at different current densities. The base-emitter voltage of a third transistor which has a negative temperature coefficient can be combined with the positive temperature coefficient voltage to produce a composite voltage having a very low or zero temperature coefficient.
The differential amplifier voltage compensator can provide temperature compensation, with a high gain loop and the differential amplifier operating together to appropriately channel current as needed. The fully compensating differential amplifier voltage regulator can produce more consistent temperature tracking than the P-N-P type design. It is still desirable, however, to extend the temperature range of the fully compensated differential amplifier voltage regulator.
An additional problem with bandgap generators is that they are susceptible to radiation. When a bandgap generator is irradiated, leakage can occur particularly at high impedance nodes and can cause large shifts in the resultant output reference voltage. It is desirable to harden the fully compensated differential amplifier voltage regulator to decrease its radiation susceptibility.